The present invention relates to a drive transmission for vehicles, which is interposed in a driving power transmission line extending from a driving power source to driving wheels. More specifically, the present invention relates to a drive transmission for vehicles that is equipped with a hydrostatic transmission (hereinafter referred to as HST) and a mechanical transmission.
The drive transmission equipped with the HST and the mechanical transmission is well-known as disclosed, for example, in an unexamined Japanese Patent Publication No. 13131/1979.
The drive transmission as disclosed in the above publication is constructed so that all the driving force from the driving power source is input to the mechanical transmission through the HST, in which the mechanical transmission stepwisely varies the speed and the HST non-stepwisely varies the speed within each speed stage of the mechanical transmission.
However, the prior drive transmission is disadvantageous in the fact that all the driving force from the driving power source is input to the mechanical transmission through the HST, for which both the mechanical transmission and the HST need to have sufficient capacities to receive the maximum output of the driving power source. Consequently, the downsizing of both the mechanical transmission and the HST is hardly made.
The present invention has been conceived to solve the above problem. It is an object of the present invention to provide a drive transmission equipped with the HST and the mechanical transmission that can be made as compact as possible.
To achieve the above object, there is provided a drive transmission for vehicles which is interposed in a driving power transmission line extending from a driving power source to driving wheels, which includes a main driving shaft, an HST, a mechanical transmission and a planetary gear device.
The main driving shaft is operatively coupled to the driving power source.
The HST includes a hydraulic pump with an input shaft and a hydraulic motor with an output shaft, in which at least one of the hydraulic pump and the hydraulic motor are of a variable-displacement type that includes a volume varying operation member. The input shaft is adapted to receive a part of driving power of the driving power source, which is branched from the main driving shaft, and the output shaft is adapted to output the driving power variable in response to the operation of the volume varying operation member.
The mechanical transmission includes a driving shaft and a driven shaft disposed in parallel with the driving shaft so as to stepwisely vary the speed between both shafts. The driving shaft is adapted to receive a part of the driving power from the driving power source via the main driving shaft.
The planetary gear device includes a sun gear non-rotatably coupled to the output shaft of the HST, a carrier non-rotatably coupled to the driven shaft, planet gears in mesh with the sun gear to be moved therearound according to the rotation of the carrier, and an outer ring with an internal gear in mesh with the planet gears, in which the driving power to be transmitted to the driving wheels is taken out of the outer ring.
According to the drive transmission of the above arrangement, the driving power of the driving power source is branched to the HST and the mechanical transmission, thereby reducing the load applied to the HST and the mechanical transmission, and hence accomplishing the downsizing of the HST and the mechanical transmission as well as the prevention of malfunctions of the transmission.
Since the mechanical transmission receives only the part of the driving power from the driving power source, the downshift or upshift speed variable of the mechanical transmission can be limited as compared with the prior drive transmission, even if the mechanical transmission is shifted to a different speed stage during the vehicle runs.
Preferably, the drive transmission further includes a driving speed changer for operating the volume varying operation member; and a detecting device for detecting the manipulated variable of the driving speed changer, so that the mechanical transmission is shifted to a different speed stage based upon signals from the detecting device.
According to the above arrangement, a single driving speed changer is enough to operate both the HST and the mechanical transmission. This can improve the operationality of the drive transmission.